Now that is an unreliable watch! It'll usually lose maybe a minute a day which is actually pretty decent for something from when Khrushchev was in power, but it likes to randomly stop or occasionally start running fast or slow according to its mood. I'm not sure how much of it is because it's a Soviet frankenwatch and how much is that it's hard to find people who'll work on Soviet watches in the UK.
They look really cool, although for the price he's asking (around 350€), I'd almost rather they use quartz movements despite the hit to historical accuracy, I don't think a 350€ mechanical chronograph can really be trusted.
If your Seiko is really drifting by minutes per day, something's badly wrong with it and you should get it serviced.
Why do you put up with that?